Keep them inside until a couple of days after vaccinations. As its so cold at the moment make sure they can get back in and have a ready supply of food to keep their energy up. Once they reach 5months keep an eye on them for signs of coming into season early as you will need to keep them inside (and away from each other if they are opposite sex).
My little one came into season early and I had to keep her indoors for two weeks, she was climbing the walls by the end of it!
I quite enjoyed it when my kitten was too young to go out as I actually got to see her, unlike now cos she's out strutting her stuff around the neighbourhood!